Dysfunction of pancreatic beta cells and loss of the functionally mature beta cell phenotype are key steps in the development of type 2 diabetes. The mature beta cell phenotype is controlled by a core transcription factor network, key nodes of which must be expressed to maintain functionally mature beta cell identity. Loss of mature beta cell phenotype under diabetic stress is marked by reduced expression or activity of transcription factors in this network. Preliminary results from our group show that the promoters of many genes differentially regulated during the acquisition of the mature beta cell state are enriched in binding sites for the transcription factor Tcf7l1. We further found that the expression of Tcf7l1 is negatively correlated with that of the mature beta cell transcription factor network: Tcf7l1 is down-regulated during beta cell maturation, and it is re-expressed in beta cells from obese-diabetic mice. Our bioinformatic analyses imply that Tcf7l1 directly represses several mature beta cell transcription factors. We created beta cell-inducible Tcf7l1 mice, and our analyses using this system demonstrate that forced expression of Tcf7l1 in beta cells results in loss of mature beta cell markers and concomitant glucose intolerance. The overarching hypothesis of this proposal is that Tcf7l1 expression must be silenced to establish full beta cell maturation, and that its re-expression under diabetogenic stress leads to the loss of the mature beta cell phenotype. Reinforcing the mature beta cell transcription factor network will be a novel strategy for preserving mature beta cell phenotype in diabetics. This could be achieved by overexpressing several nodes of the network, but, given our observation that Tcf7l1 likely is antagonistic to the mature beta cell transcription factor network, a more viable strategy may be the downregulation of Tcf7l1. Our first two aims will establish the connection between Tcf7l1 and the mature beta cell transcription factor network. Our third aim will look at upstream signaling pathways that can be perturbed to achieve the desired effect on Tcf7l1 levels. These studies could provide new insights into the mechanism(s) regulating the acquisition, maintenance and collapse of the mature beta cell state, and provide novel strategies to prevent its loss in diabetics.